The first three parts of the essay describe the evolution of Italian administrative law, from the foundation of the unitary state, in the second half of the 19th century, to the present time. The fourth part is devoted to the analysis of some general features of administrative law today, especially focusing on its “criterion,” i.e. its borders and its distinction from other provinces of law. In the 19th century both the unitary State and the administrative law were born and developed in Italy. The features of the latter were strongly influenced by the unification process and by political events. In the first half of the 20th century administrative law underwent major developments involving the range of administrative tasks, the size of administrations, their organization and their relations with citizens. Administrative law was still affected by political developments and displayed its authoritarian facet. In the second half of the century, many trends which had begun in the former period continued and some of the problems which had arisen in the first half of the century grew more serious. The institutional context, however, was very different. The principles of the democratic Constitution gradually changed the shape of administrative law, which came to display its liberal facet. At the beginning of the 21st century, the problems of delimiting administrative law may be traced to three main profiles: that of the distinction from private law; that of the correspondence between administrative law and administrative justice and, therefore, of the apportioning of jurisdiction; that of placement within the sphere of public law.